In internal combustion engines, the fuel systems have been replaced most recently by electronically controlled fuel injection systems. Accordingly the conventional carburetor with a choke to enhance performance during cold starting is eliminated. It has been found that in large engines, such as truck engines utilizing a throttle body fuel injection system, the engine requires long cranking start times at cold temperatures, and occasional restarts due to stalling. Accordingly, it was determined that this could be corrected by opening the throttle plates called cracking, approximately 0.1 inch. However, there is a need for a device that would function as the engine temperature increases to eliminate the opening of the throttle plates at idle and thereby avoid rough idling as well as minimize the undesired fuel emissions.
In consideration of this problem, various solutions were considered. Among these were a direct actuation of the throttle plate by metal coil but this was rejected because of the large size required to develop the loads that were needed. Bi-metal snap-action discs were investigated but, they were found to be limited in force, had a limited stroke and required a stack of a plurality of discs which would result in a costly solution. Wax pellet actuators were considered but it was found that no wax was available that could provide a transition point at cold temperatures such as 32.degree. F. It was also judged that such a device would be complex and would have a large variation in the actuation temperature tolerance. Vacuum actuators were also considered which would function to extend when the engine is off and retract when engine starts. However this was found to be an expensive and bulky solution.
Among the objectives of the present invention were to provide a thermal throttle actuator that would function to maintain the throttle plates open the desired amount while at cold temperature, which would function at proper temperature tolerances to eliminate the cracking of the throttle plates at warm idle; which had minimal hysterisis; which was compact; which could readily be adapted to a throttle body; and which was relatively low in cost.
In accordance with the invention, the thermal throttle actuator is adapted to be mounted in the opening of a throttle body of an internal combustion engine wherein the fuel system is electronically controlled. The thermal throttle actuator comprises a housing in which a plunger is mounted for reciprocating movement and is adapted to contact a spring loaded throttle plate in the throttle body. The plunger is yieldingly urged toward the throttle plate by a bias spring. A shape memory alloy in the shape of a helical spring is provided to produce a force on the plunger in a direction opposite to that of the bias spring when the shape memory alloy is heated to a temperature above a predetermined temperature. When the temperature is below the predetermined temperature, the shape memory alloy spring is readily deformable and the bias spring functions to provide a force on the throttle plate which maintains the throttle plate in open position to enhance performance during cold starting of the engine.